Recently, attention has been focused on a magnetic random access memory (MRAM) which can possibly replace a conventional dynamic random access memory (DRAM). As described in U.S. Pat. No. 5,734,605, for example, the conventional MRAM employs the following recording method. In a tunnel magnetoresistive effect (TMR) element having a magnetic film/nonmagnetic insulating film/magnetic film multi-layer structure, the magnetization of one of the magnetic films is reversed with a synthetic magnetic field produced by currents flowing through two metal interconnections which are provided respectively above and below the TMR element in directions orthogonal to each other. However, even in the case of the MRAM, when the size of the TMR element is reduced for achieving a larger capacity of the MRAM, the magnitude of the magnetic field required for magnetization reversal is increased, and thus a larger amount of current needs to flow through the metal interconnections. Accordingly, problems of an increase in power consumption and breakage of the interconnections have been pointed out.
As a method of reversing magnetization without using the magnetic field, it has been theoretically shown that the magnetization can be reversed only by causing a certain or larger amount of current to flow through a giant magnetoresistive effect (GMR) film or a tunnel magnetoresistive effect (TMR) film used in a magnetic reproducing head. For example, spin torque magnetization reversal has been demonstrated by using a nano-pillar employing a TMR film, as described in Applied Physics Letters, Vol. 84, pp. 3118-3120 (2004). Having the ability to produce output equal to or more than that producible by the conventional MRAM, the spin torque magnetization reversal using a TMR film has been particularly drawing a lot of attention.
FIG. 1 illustrates a schematic diagram of the spin torque magnetization reversal mentioned above. In FIG. 1, a magnetoresistive effect element and a transistor 6 are connected to a bit line 1. The other end of the transistor is connected to a source line 7. The magnetoresistive effect element includes a first ferromagnetic layer (recording layer) 2 whose magnetization direction varies, an intermediate layer 3, and a second ferromagnetic layer (fixed layer) 4 whose magnetization direction is fixed. The conduction of the transistor 6 is controlled by a gate electrode 5. When the magnetizations of the fixed layer 4 and the recording layer 2 are to be changed from an antiparallel (high resistance) state to a parallel (low resistance) state, a current 8 is caused to flow from the bit line 1 to the source line 7, as shown in part (a) of FIG. 1. In this event, electrons 9 flow from the source line 7 to the bit line 1. On the other hand, when the magnetizations of the fixed layer 4 and the free layer 2 are to be changed from the parallel (low resistance) state to the antiparallel (high resistance) state, the current 8 is caused to flow in a direction from the source line 7 to the bit line 1. In this event, the electrons 9 flow in a direction from the bit line 1 to the source line 7.
Applied Physics Letters, Vol. 84, pp. 3897-3899 (2004) proposes a method of performing the spin torque magnetization reversal by using a magnetization perpendicular to the magnetic film plane without changing the current directions. As shown in FIG. 2, this example uses a magnetoresistive effect element obtained by stacking a fixed layer 21 having a magnetization direction perpendicular to the film plane, a nonmagnetic first intermediate layer 22, a recording layer 23 having a magnetization direction in the film plane, a nonmagnetic second intermediate layer 24, and a reference layer 25 having a magnetization direction in the film plane. For example, when the magnetization of the recording layer 23 is parallel to the magnetization of the reference layer 25, a positive current 26 is first caused to flow to apply spin torque to the magnetization of the recording layer. The time length for which the positive current is caused to flow is a time length corresponding to ¼ of a cycle T of the precession of magnetization induced by the spin torque. The current direction is then switched to an opposite direction 27 to apply torque to stop the magnetization movement. Thus, this example shows that the spin torque magnetization reversal is possible in a time length corresponding to only T/2.
JP 2006-128579A describes using a magnetoresistive effect element as shown in FIG. 3 including a fixed layer 31 having a magnetization in a direction in the film plane, a nonmagnetic layer 32, a free layer (recording layer) 33 having a magnetization in a direction in the film plane, a nonmagnetic layer 34, and a spin torque driving layer 35 having a magnetization perpendicular to the film plane. This document discloses that, only by causing a current 8 to flow, the magnetization direction of the free layer 33 can be reversed to whichever direction, i.e., from a direction parallel to the magnetization direction of the fixed layer 31 to a direction antiparallel thereto, or from the direction antiparallel to the magnetization direction of the fixed layer 31 to the direction parallel thereto. The document also discloses that whether to perform the magnetization reversal is controlled by controlling pulse time.